1. Field of the Invention
This invention pertains generally to the efficient employment of a swath survey system and more particularly a swath survey system wherein the swath surveys are automated.
2. Description of the Related Art
The traditional goal when surveying an ocean bottom has been primarily to provide the mariner with the data needed to ensure safety of ship navigation. Bottom contours and sparse selected soundings are generally sufficient to meet this need. However, with increasing demands on the accuracy of hydrographic surveys and accelerating commercial exploitation of the sea floor, data is desired that will provide more than a general characterization of the sea floor. Modern requirements demand 100% coverage, i.e., coverage that provides a dense set of soundings suitable for generation of a gapless topographic representation of the sea floor.
Modern hydrographic sounding systems are capable of meeting this need, but the environment significantly impacts their performance. Because of the complexity of the environmental effects, in-situ assessment of system performance is required to ensure 100% coverage.
For many decades surveys have been primarily conducted using vertical single-beam sonar systems. Since it is impractical to achieve 100% coverage with these systems, surveys are conducted using a series of preplanned lines that are based on typically scant historical knowledge of an area""s depth contours. Acoustic imaging systems are used to ensure that shallower areas do not exist between the sounding lines. These imaging systems provide wide area bottom coverage but do not yield sufficiently accurate depth soundings for charting purposes, and the generated images typically require human interpretation. When questionable areas are found in the imagery, the single-beam system is deployed over the area for accurate soundings.
In contrast, modern swath systems use multibeam sonar technology. These systems provide multiple soundings with each sonar ping that are located within a wide swath perpendicular to a ship""s track. When properly compensated, all of the soundings generated can achieve the required accuracy needed for charting and other purposes. As compared with single-beam systems, swath systems can provide 100% bottom coverage, yielding denser soundings and faster coverage of the area. Even though swath system hardware cost is much higher than single-beam, the ability to achieve rapid total bottom coverage allows these systems to be more cost effective for charting.
Swath systems can provide superior performance, achieved through significant added complexity in the survey system and its operation. The effective sea floor coverage and accuracy of a swath system is principally affected by several factors: ocean depth, positioning errors, ray bending, and bottom type and morphology.
Swath systems are typically operated at or near the ocean surface in order to maximize bottom coverage with time. Since a swath sonar covers an angular sector (as larger as 150 degrees for some commercially available systems) the actual swath width on the ocean floor varies with ocean depthxe2x80x94narrower in shallow water and wider in deep water.
Swath sonar systems provide range as a function of angle with respect to the sonar head. To generate soundings from this data accurate measurements of sensor pitch, roll, heading, heave, and position (vertical and horizontal) are required. The affect of pitch, roll and heading errors are most severe in the outer beams of the system due to the greater slant range. The result of such errors is to reduce the usable systems swath width.
Sea state and sea direction can adversely affect system performance. Rough seas can exceed the capability of the pitch, roll, heading, and heave sensors to correctly compensate the sonar data. Consequently, sea direction becomes significant since the vessel will handle differently depending on its heading relative to the seas. High sea states can also result in aeration of the water under the sonar head which can drastically reduce effective range and swath width, and this effect will vary with time and heading.
The sound velocity structure of the water column affects the direction sound travels through the water, resulting in ray bending. The consequences of this for a swath system is uncertainty in the proper location of the bottom, particularly in the outermost beams.
Bottom composition affects the return strength of the sonar pulse and thus the effective range and swath width of the system.
Bottom morphology can have several affects on swath system performance. Sand waves can result in destructive interference of the acoustic signals. Proud bottom features can mask low-lying areas. Excessive slope can affect the ability of the system to track the bottom and affects return signal strength.
The significant consequence of these combined factors is that it is difficult to predict a-priori the effective swath width of a multibeam sonar, making it impractical to pre-plan survey lines to achieve minimum survey time while ensuring complete bottom coverage. Consider a particularly simple case, where a series of parallel lines are to be run over an area with a slope, and the lines are oriented perpendicular to the contour of the slope. This might be necessary due to weather or sea state. If planned line spacing is computed using the average depth and the nominal swath width, the result will be excessive overlap between swaths in the deep areas (wasted survey time) and gaps between swaths in the shallow areas (missing data). The missing data are called xe2x80x9cholidaysxe2x80x9d and the areas of excessive overlap are called xe2x80x9coverages.xe2x80x9d
Therefore, it is evident that the current techniques have has two fundamental drawbacks; first, the swath system performance, in terms of swath coverage width, is significantly affected unpredictable environmental conditions, and second, parallel survey lines will result in data gaps (holidays) and overages (excessive coverage) in areas where the ocean floor has significant morphology. Holidays result in loss of data and overages result in wasted survey time.
In a normal survey practice data is collected and the achieved data quality and coverage is analyzed after completion of the survey. Existence of holidays and overages, considering only quality data (data that meets some defined quality constraint), are not apparent until after the survey is completed. At this point the time loss due to overages cannot be recovered, and the survey vessel must be re-deployed to recover the data over the gaps.
The object of this invention is to achieve 100% survey coverage with survey data of acceptable quality in minimum time.
Another objective of the invention is reduced human operator requirements.
Another objective is to provide a simulation capability that will allow prediction of system performance over an area given pre-existing data.
These and other objectives are achieved in the autonomous survey system (AutoSurvey) by evaluating the effects of the environment and system performance on the collected data. The AutoSurvey system accomplishes this by modulizing the data collection into a series of modules xe2x80x94data collection and error detection, data georectification, data quality validation, swath-edge fit, next-line way point generation, and the autopilot. All of these processes are implemented in near real-time, allowing unfettered survey progress. The data is applied directly between processes, providing operator independent system operation; the AutoSurvey system directly controls the survey vessel via the autopilot. Through the real-time data acquisition the system provides automation of the operator quality and coverage assessment tasks and also provides quantified data. The operator is able to adjust the system operating parameters to compensate for ambient conditions and to determine subsequent navigation way points as a function of the specified survey criteria.